1. Field of the Invention
The present invention relates to a method of selecting a critical plane for use in predicting fatigue life of an object experiencing multiple stress inducing events.
2. Background Art
Uniaxial fatigue theory is a fatigue life prediction tool. It is used to predict the ability of an object to withstand repeated exposure to stresses before cracking. The process is referred to as xe2x80x9cuniaxialxe2x80x9d because the prediction is based on one-dimensional stresses being induced within the object.
Critical plane theory is a method to model multi-dimensional stresses as one-dimensional stresses. The one-dimensional stresses can then be analyzed using uniaxial fatigue theory to predict fatigue life. The one-dimensional stresses are determined by selecting a critical plane direction and determining how much stress the multi-dimensional stress exert in the direction of the critical plane.
A problem with the critical plane approach arises when the object experiences a number of stress inducing events. In the past, if there were three events under consideration, three damage calculations would occur relative to three different critical planes. The fatigue life would then be determined by adding together the three damage calculations. This is a problem because the damage calculations are relative to three different critical planes. Accordingly, there exists a need for a method of selecting a critical plane so that the critical plane can be used for the damage calculations of all events. In this manner, damage calculations from each event can be added together for the purposes of calculation fatigue life using uniaxial fatigue theory.
The present invention overcomes the foregoing problem with a method for selecting a critical plane. The method analyzes stress induced by multiple events to select a critical plane. The critical plane is suitable for use in calculating damages caused by each event. The present invention thus eliminates the problem of adding together damage calculations made relative to different critical planes.
One aspect of the present invention relates to a method for use with an object experiencing multiple stress inducing events. The method comprises determining a stress distribution histogram for each event and superimposing each stress distribution histogram to create a superimposed stress distribution histogram. A critical plane is determined based on the superimposed stress distribution histogram.
One aspect of the present invention relates to a system for predicting fatigue life of an object experiencing multiple stress inducing events. The system comprises a computer which determines a stress distribution histogram for each event. The stress distribution histograms are superimposed to create a superimposed stress distribution histogram so that the computer can select a critical plane from the superimposed stress distribution histogram.
One aspect of the present invention relates to a method for predicting a fatigue life of an object experiencing multiple stress inducing events. The method comprises determining a stress distribution histogram for each event and weighting at least one of the stress distribution histograms. The method further comprises superimposing each stress distribution histogram to create a superimposed stress distribution histogram. A critical plane is calculated based on the superimposed stress distribution histogram. Once that critical plane is calculated, the stresses induced by each event are adjusted relative to the critical plane. The largest of the adjusted stresses is determined. New events are determined by shifting each event according to the largest stress. This provides mean stress correction. The damage caused by each new event is calculated. The damage associated with the largest stress is subtracted from each of the damage calculations to determine a corrected damage for each of the new events. The corrected damages are added together to determine a total damage. The total damage is used to predict the fatigue life of the object.